How Do Deodorants Compare on Bacterial Counts and Odor? A PQ-16 Study

To evaluate a deodorant, it is necessary to investigate not only whether bacterial numbers fall, but also whether the odor perceived by the human nose becomes weaker.

In a study of a roll-on formulation containing polyquaternium-16 (PQ-16), underarm odor was rated lower after 24 and 48 hours than with the commercial alcohol-based formulation used for comparison. However, odor scores did not differ clearly from those of a formulation containing an aluminum salt in the same base. The research examined both bacterial counts and odor, including how the findings depended on the comparator.

Evaluating a new antimicrobial ingredient on actual underarms

The paper by Traupe and colleagues was published online in 2016 and appeared in a 2017 journal issue. Its principal aim was to test in people how a formulation containing the cationic polymer PQ-16 affected underarm bacteria and odor. Skin condition during use was also observed.

Underarm odor involves substances produced when skin bacteria break down secretions. Some cationic substances act on bacterial membranes, a property that attracted the authors’ attention. The paper describes how PQ-16 was selected through preliminary comparisons of candidate polymers. The main human trials, however, assessed the complete formulation incorporating PQ-16.

Comparing three formulations on opposite underarms

The test product was a fragranced microemulsion roll-on containing 1% active PQ-16. There were two comparators: a commercial alcohol-based roll-on that then claimed 24-hour odor protection, and a formulation with 10% aluminum chlorohydrate (ACH) instead of PQ-16 in the same microemulsion base.

ACH is also used in antiperspirant formulations, but here it was a comparator for antimicrobial and deodorant performance. Sweat quantity was not measured. All three formulations contained fragrance, and the bases of the PQ-16 and ACH formulations also contained piroctone olamine. The design therefore did not isolate PQ-16 alone as the cause of differences from the commercial product.

Participants used a designated unscented cleanser beforehand and refrained from using deodorants and similar products. Approximately 500 mg of the PQ-16 formulation was applied to one underarm and the comparator to the other, with sides randomly allocated. Formulation identities were concealed, and soap and deodorizing products were restricted on the underarms until measurement.

Bacterial counts and odor were examined in separate trials

The bacterial-count trials included 17 people in the commercial-product comparison and 20 in the ACH-formulation comparison. Bacteria were collected from a defined underarm area before application and 1, 4, 8, 24, and 48 hours afterward, then cultured and counted as colonies. One measurement was missing at 48 hours in the ACH comparison, leaving 19 people for that analysis.

The odor trials recruited 35 people for the commercial-product comparison and 38 for the ACH comparison. Six trained assessors directly smelled the underarms and scored odor intensity on a six-level scale from 0 to 5.

Baseline odor was first measured 24 and 48 hours after washing alone. Washing and product application then took place, followed by another assessment at 24 and 48 hours. Because inclusion in the analysis required a minimum baseline odor, the commercial-product comparison included 28 people at 24 hours and 34 at 48 hours; the corresponding numbers for the ACH comparison were 29 and 31. The same full set of participants was therefore not analyzed at every time point.

Lower odor scores than the commercial product at 24 and 48 hours

Compared with the commercial alcohol-based formulation, mean odor scores on the PQ-16-treated side were 1.98 versus 2.48 after 24 hours and 2.45 versus 2.95 after 48 hours. The PQ-16 side was 0.5 points lower at both times, with statistically significant differences.

In the ACH comparison, scores for the PQ-16 and ACH formulations were 1.30 versus 1.31 at 24 hours and 2.17 versus 2.05 at 48 hours. Neither difference was significant. Relative to the washing-only baseline, both the PQ-16 and ACH formulations had lower odor ratings at 24 and 48 hours.

These results show that the PQ-16 formulation reduced odor more than the commercial comparator under the conditions tested. For the ACH formulation, the finding was that no difference was detected; this was not an equivalence trial establishing that the products perform equally under all conditions.

Bacterial-count differences depended on comparator and time

For bacterial counts as well, changes from the pre-application level were lower with the PQ-16 formulation than with the commercial product at every measured time point. With the commercial product, bacterial counts initially fell but exceeded pre-application levels after 24 and 48 hours.

Compared with the ACH formulation, bacterial counts on the PQ-16-treated side were significantly lower at four and eight hours. Differences at one and 24 hours were not clear, and the values were nearly the same at 48 hours.

The period during which bacterial counts with PQ-16 were significantly below the pre-application level also differed between comparison groups. Both groups showed an initial decrease, but a clear decrease lasting through 24 hours occurred only in the group compared with the commercial product. Being lower than a comparator at 48 hours and remaining below the pre-application level for 48 hours are different findings.

Reading a deodorant trial in the context of formulation and conditions

One feature of this study is that it did not use bacterial counts alone as a substitute for deodorant efficacy: it also included odor evaluations by trained people. In the PQ-16-versus-commercial-product comparison, both changes in bacterial counts and odor scores pointed in the same direction, with lower values on the PQ-16 side.

However, there was no untreated underarm control followed over the same period. Comparisons with pre-application values therefore also include the effects of washing and elapsed time. Moreover, these were fragranced, multi-ingredient formulations, and the comparator was a particular product available at the time. The findings cannot be extended to current products from the same brand or to every product bearing the same ingredient name.

No incompatibility reactions or discomfort were observed or reported during the trials. This finding applies to the participants and period studied; it does not establish safety over long-term use or for every skin type.

Summary

The test formulation containing 1% PQ-16 had underarm odor scores 0.5 points lower at 24 and 48 hours than the commercial alcohol-based formulation, and changes in bacterial counts were also lower at each measurement time. Compared with the 10% ACH formulation in the same base, odor scores did not differ significantly, while clear bacterial-count differences were confined to four and eight hours.

Measuring both bacterial numbers and human-perceived odor allowed specific comparisons of formulation performance. When interpreting deodorant findings, it helps to consider not just the ingredients, but also the comparator formulations, washing conditions, and the number of hours before measurement.

The study was fully funded by Beiersdorf AG. The paper lists all four authors as affiliated with the company’s research and development division.

References

Traupe B, Fölster H, Max H, Schulz J. Effective axillary malodour reduction by polyquaternium-16-containing deodorants. International Journal of Cosmetic Science. 2017;39(2):141–148. doi:10.1111/ics.12358. Open-access full text.